1. Field of Invention
The present invention relates to a method for fabricating a device with a flexible substrate. More particularly, the present invention relates to a method for fabricating a device with a flexible substrate which can be easily stripped from the rigid substrate.
2. Description of Related Art
As well known, an electronic product includes a lot of electronic devices, such as transistor device. These electronic devices are usually formed on a substrate. Taking the conventional technology in semiconductor fabrication as the example, usually, a silicon substrate is used as a substrate for fabricating the electronic devices. Particularly, almost all of the transistor fabrications take the silicon substrate as the substrate in fabrication. However, the silicon substrate is a rigid substrate, and therefore the device being formed is also a rigid device, without being flexible. The electronic product is then not bent.
As the technology is continuously developed, some electronic devices can be formed from organic material. For example, the technology about the organic thin film transistor (OTFT) has gradually been well developed. For the available organic materials, the materials are generally divided into two types, for example, material with small molecules, such as pentacene, or material with large molecules, such as polymer.
In comparing the OTFT with the conventional in-organic transistor, the OTFT can be fabricated under a relative low temperature, therefore the substrate can be selected from plastic or glass, which are light, thin, and cheap. In addition, the fabrication process for the OTFT is simpler, and can use the printing technology to directly pattern the organic thin film. This can decrease the number of masks being used and the vacuum deposition equipments. Further, since the OTFT is suitable for fabrication with the plastic substrate and is compatible with the process of roll-to-roll, it is quite helpful to reduce the fabrication cost in the future.
In order to form device on the flexible substrate, it should be overcome on the issues that the temperature and stress may cause deformation on the substrate and then occurrence of misalignment. Some conventional issues are described as follows.
FIGS. 1A-1C are cross-sectional views, schematically illustrating the conventional fabrication process for the OTFT. In FIG. 1A, a rigid substrate 100 is provided to serve as the fabrication substrate. In addition, a flexible substrate 104 is adhered to the rigid substrate 100 by an adhesive layer 102. The flexible substrate 104 is used for fabricating devices thereon.
Then, in FIG. 1B, a device structure, such as an OTFT, is to be formed on the flexible substrate 104. The fabrication process includes, for example, a gate electrode 108 is formed on the flexible substrate 104. A dielectric layer 106 is formed over the flexible substrate 104 and covers the gate electrode 108.
It should be noted that since the fabrication processes are performed under a temperature. For example, all of the material layers, including the adhesive layer 102, may cause a bending, such as bending up as shown in FIG. 1B, due to difference of the thermal expansion coefficient. As a result, the requirement of alignment in fabrication device may have the misalignment for the position of the gate electrode 108.
Further in FIG. 1C, when the flexible substrate 104 is stripped from the rigid substrate 100, due to the adhering force from the adhesive layer 102, the flexible substrate 104 is not easy to be stripped, and it needs to apply a stress on it, so as to be stripped. In addition, since the adhesive layer 102 in the fabrication process is still under the temperature of about 200° C., and the adhesive material could be transformed and cured into different material, and then a residue 102a may remain on the back of the flexible substrate 104. This would cause the poor quality of the device, or even damage the device. Moreover, the adhesive material is necessary to be coated and then adhered to the rigid substrate, the planarity of the substrate cannot be easily controlled, and the fabrication processes are complicate. The poor planarity of the substrate may also cause the issue of misalignment.
For another conventional technology is shown in FIGS. 2A-2C. Referring FIG. 2A, in order to reduce the amount of use for the adhesive material, the adhering tapes 116 are only on the four corners, for adhering the flexible substrate 104. As a result, a space gap 118 exists between the flexible substrate 104 and the rigid substrate 104. FIG. 2B is a top view in FIG. 2A. The adhering tapes 116 are on the four corners of the flexible substrate 104.
In FIG. 2C, the subsequent fabricating processes for the OTFT include, for example, forming the gate electrode 122, the dielectric layer 104, source/drain regions 124 and 126, and the channel layer 128. This conventional method can simplify the process of stripping the flexible substrate 104. However, since the issues about the thermal expansion coefficients for the flexible substrate and the rigid substrate being different severely exits, it cause the misalignment problems on the device being fabricated.
In above considerations, the conventional technology still does not propose the proper flexible substrate 104 for fabricating the device thereon.